The sperm of some species of fruit fly are up to 5.8 centimetres long — around 20 times as long as the fly itself (pictured, two Drosophila bifurca sperm spill out of the male's ruptured seminal vesicle). Giant sperm tails are energetically expensive to produce, and their evolution requires intensive sexual selection. And yet theory predicts that sexual selection should steadily weaken as sperm get larger and their numbers decline. On page 535, Lüpold et al. investigate this 'big-sperm paradox' (S. Lüpold et al. Nature 533, 535–538; 2016).
Longer sperm are better than their shorter comrades at displacing competitors from the female's seminal receptacle, and the length of the seminal receptacle determines the female's preference for longer sperm. The authors demonstrate that the genes that confer longer sperm are correlated with those that confer a longer seminal receptacle. Thus, female preference and sperm size coevolve. This process is reinforced by the fact that a longer seminal receptacle correlates with shorter intervals between female mating, increasing sperm competition and hence the advantage to longer sperm.
But what is the benefit to females? Only large, healthy males that carry 'good' genes can produce long sperm in sufficient quantities to outcompete other suitors. Therefore, a long seminal receptacle maximizes a female's chances of producing offspring that have high-quality genes. Both sexes win in this evolutionary arms race.
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Philosophical Transactions of the Royal Society B: Biological Sciences (2020)